NiMh batteries pop out

[Bill]

Trying to charge new batteries and after a couple of minutes they and the
charger jumped off the wall. Happened twice.
I would blame the new batteries but it happened with older ones too on one
occasion.
I'm pretty sure the polarity was as instructed.
What could be the cause and correction? Thanks

[ASAAR]

I've had batteries pop out certain chargers, and it's the fault of
charger (poor design), not the batteries. On the other hand, I've
never had the charger jump off the wall. That could be eliminated
by plugging the charger into a cheap, short extension cord. Unless
you replace the charger, if yours is similar to the ones I've used
that encouraged batteries to pop out, a small rubber or elastic band
would keep the batteries secure. The polarity shouldn't have been a
problem. Take a closer look at the charger. Unless it's fairly
old, it will be designed such that if a battery is inserted
backwards, the battery's base (negative terminal) will not make
electrical contact with the charger's circuit. The battery won't
get charged, but it's protected from being damaged. Even if the
battery is inserted backwards and you don't notice it, one of the
charging LEDs should fail to light. This reminder isn't always
possible with cheap chargers, that use a single LED to display the
state of multiple charging circuits.